Fluoroelastomers having excellent heat resistance, oil resistance, and chemical resistance have been used widely for sealing materials, containers and hoses. Examples of fluoroelastomers include copolymers comprising units of vinylidene fluoride (VF2) and units of at least one other copolymerizable fluorine-containing monomer such as hexafluoropropylene (HFP), tetrafluoroethylene (TFE), chlorotrifluoroethylene (CTFE), vinyl fluoride (VF), and a fluorovinyl ether such as a perfluoro(alkyl vinyl ether) (PAVE). Specific examples of PAVE include perfluoro(methyl vinyl ether), perfluoro(ethyl vinyl ether) and perfluoro(propyl vinyl ether).
In order to develop the physical properties necessary for most end use applications, fluoroelastomers must be crosslinked (i.e. cured). A preferred curing system for many end uses is the combination of an organic peroxide and a multifunctional unsaturated coagent. The coagent forms crosslinks by reacting with bromine or iodine atom cure sites on the fluoroelastomer polymer chain. A preferred cure site is an iodine atom bonded to a carbon atom on the fluoroelastomer chain.
Typical peroxide curable fluoroelastomer compositions also contain fillers such as carbon black, SiO2, TiO2, BaSO4, Al2O3, etc. Fillers generally improve the tensile properties of the cured fluoroelastomer. In the case of peroxide curable fluoroelastomer compositions, fillers also improve the processability of the compositions on rubber manufacturing equipment such as mills and extruders where slippage may occur in the absence of filler, and in compression molding where mold fouling can occur without filler present in the compositions. However, fillers may also be an undesirable source of contamination by metals or particles in some end use applications such as in semiconductor manufacturing equipment where elastomer articles contact harsh environments such as plasmas.
Unfilled peroxide curable fluoroelastomer compositions are known in the art, but they are typically difficult to manufacture and have low tensile properties, making them unsuitable for some end use applications.
Curable compositions of fluoroelastomers that contain particles of dynamically cured fluoroelastomer particles (also referred to as gel) and filler have been previously disclosed, e.g. U.S. Pat. No. 4,708,988. The presence of gel appears to decrease the tensile strength of the peroxide cured compositions.